Every laser is attracted to one thing. With laser hair removal the laser is attracted to pigment (or melanin). Gray hair has zero pigment, blonde having very little. Most blondes have very light hair on their face, arms and legs but for many their bikini and underarm hairs have more pigment. As long as the hair looks a little darker there is a good chance that we will be able to laser them.
Sometimes what happens is the laser is able to pick up most or all of the hair at the beginning but as we go through the treatments the hair continues to get lighter and finer. We may get to a point after a handful of treatments that we are not able to get all of the hairs (the ones that are now too light and fine). Some women do not mind not completely getting rid of all of the hair as log as they have thinned out all of the rest.
As in any laser hair removal treatment we can never guarantee results. We can laser the hair but it is how your hair reacts to the laser and also what is happening in your body (as far as hormones, medications etc) that ultimately controls the results.